Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. Ambient air may be supplied to the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy to the working fluid (air) to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel nozzles into a combustion chamber in each combustor where the compressed working fluid mixes with fuel and ignites to generate combustion gases having a high temperature and pressure. The combustion gases expand in the turbine to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Various design and operating parameters influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flame holding conditions in which the combustion flame migrates toward the fuel being supplied by the fuel nozzles, possibly causing accelerated wear to the fuel nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, a plurality of tubes may be radially arranged in a cap assembly to provide fluid communication for the working fluid and fuel through the cap assembly and into the combustion chamber. Although effective at enabling higher operating temperatures while protecting against flame holding and controlling undesirable emissions, some fuels and operating conditions produce very high frequencies in the combustor. Increased vibrations in the combustor associated with high frequencies may reduce the useful life of one or more combustor components. Alternately, or in addition, high frequencies of combustion dynamics may produce pressure pulses inside the tubes and/or combustion chamber that may adversely affect the stability of the combustion flame, reduce the design margins for flame holding, and/or increase undesirable emissions. Therefore, a system that reduces resonant frequencies in the combustor would be useful to enhancing the thermodynamic efficiency of the combustor, protecting the combustor from accelerated wear, promoting flame stability, and/or reducing undesirable emissions over a wide range of combustor operating levels.